Spark plug



Jan. 23, 1945. M. F. PETERS ET AL SPARK PLUG Filed Sept. 13, 1943 @,:iv m M la?, W//d/d//f/ W JOHN ELLSWGRTH D. NICCARTY H01 'nuff Patented Jan. 23, 1945 SPARK PLUG Melville F. Peters, Beltsville, Md., and John Ellsworth D. McCarty, Jackson Heights, N. Y.; said McCarty assignor to The Firestone Tire & Rubber Company, Akron, Ohio, a corporation of Ohio Application September 13, 1943, Serial No. 502,388

(Cl. 12B-169) Claims.

This invention relates to spark plugs for internal combustion engines, and more especially it relates to spark plugs adapted to withstand severe temperature changes and high temperatures for long periods of time.

The invention is of especial utility in its application to spark plugs for airplane engines wherein the aforementioned severe service conditions usually obtain. The high heat in spark plugs developed by engine operation is very difficult to dissipate and in some instances, the center electrode becomes hot enough to burn the insulation from the high tension wire secured to the terminal at the upper end thereof, in addition to burning away the sparking end of the electrode. Furthermore, the strains placed upon the porcelain element by the extreme heat may cause the cracking or breaking of the porcelain with resulting loss of compression in the engine cylinder.

The chief object of this invention is to provide an improved spark plug capable of efficient and satisfactory operation under severe service conditions. More specifically, the invention aims to provide in an improved manner for the dissipation of heat in the center electrode; to increase the rate of heat transfer from the porcelain of the spark plug to the metal shell in which it is mounted; to provide means by which the temperature of the spark plug may be controlled; to provide a spark plug construction wherein the maximum mechanical stresses and the maximum thermal gradients occur at different regions of the plug; and to provide spark plug construction wherein unequal or local stresses on the porcelain are obviated. Other objects will be manifest as the description proceeds.

The single iigure of the accompanying drawing shows a spark plug embodying the invention, partly in elevation and partly in longitudinal diametric section.

Referring to the drawing, there is shown a spar p ug comprising the usual metal shell I0, a porcelain insulator II mounted therein, an axially disposed center electrode I2 carried by the porcelain and projecting from both ends thereof, and a ground electrode I3 carried at the lower .end of the shell I0 and projecting radially thereof toward the center electrode I2. The center electrode I2 lits snugly in a. bore formed in the lower half of the porcelain II, the upper half of the porcelain being formed interorly with screw threads to recive a complementally threaded portion I2a of said electrode, the arrangement being such as to provide gas-tight engagement of porcelain and electrode. portion of the electrode I2 is threaded at I2b to receive the usual nut I4 by means of which a wire (not shown) of the ignition system of a motor may be connected to the plug.

The porcelain II is formed exteriorly at its medial region with a circumferential flange or bulge I6, which flange is received within a recess I'I in the metal 'shell I0, the lower end of said recess being defined by an annular ledge or shoulder I8. The salient feature of the invention is a thin metal girth sheath I9 on the outer face of the porcelain I I, said sheath covering the flange or bulge I6 of the porcelain, and extending above and below the same. The arrangement is such that when the porcelain is operatively mounted in the shell Ill, the metal sheath I9 extends somewhat beyond the top of the shell IIJ, and somewhat below the shoulder I8 therein, the bulge I6 of the porcelain being centrally disposed in the recess I1 of the shell I0. The porcelain is securely held in place in the shell by vmeans of a pair of annular metal gaskets 20, 20 that are snugly received in the recess I'I, and have inner circumferential faces shaped complemental to the peripheral surface of the metalclad portion of the porcelain I I so as to fit substantially flush thereagainst. The lower gasket 20 rests upon the shoulder I8, and the upper end of the shell I0 is crimped at 2l over the upper gasket 20 so as permanently to secure the elements of the plug in properly assembled relation. The shape of the gaskets 20 is such that they have wedging engagement with the porcelain II and the peripheral face of the recess I'I.

The ideal composition for the porcelain sheath I9 is a metal, such as platinum, iridium or the like that has the same low thermal expansion as porcelain, but these metals are too rare and expensive to be practical. Accordingly, copper has been selected as most nearly satisfactory since it is a good conductor of heat, is soft enough to yield readily under strain, such as will arise due to differences in thermal expansion, is available in sufficient quantity, and is not expensive. The copper is applied 'to the porcelain by plating, coating, or other suitable means whereby the sheath is in one piece and without seams. The thickness of the copper sheath affects the heat transfer characteristic thereof, so that in practice the copper plating will .be of optimum thickness in View of the function to be performed thereby. The gaskets 2U are of softer metal than the steel of the shell I0, and advantageously may be composed of copper or brass.

The upper terminal In use, the high heat of the center electrode I2 is transferred by conduction to the porcelain II, thence in order to the metal sheath I9, the metal gaskets 20, and the shell IIJ, finally being dissipated in the cylinder head (not shown) in which the plug is mounted. The rate of heat transfer from the porcelain I I is increased many fold over conventional spark plugs since the metal sheath and metal gaskets 2|! greatly increase the area of the metal path from porcelain II to shell I0. By varying the length and/or the thickness of the sheath IB it is possible to control the heat of the plug.

The invention also is advantageous in that fewer porcelains will be damaged during assembly of the plugs, and during use thereof. Porcelains commonly are not straight or true due to the process of forming and firing the same. Such porcelains are subject to unequal or local stresses during assembly with the shell, and subsequently due to thermal expansion, and as a result frequently are fractured. Such porcelains when metal-clad according to the invention Will not readily fracture since the soft metal sheath will flow and equalize the stresses to which the porcelain is subjected, and also will function to dissipate heat so that the-extent of thermal expansion is reduced. Furthermore, the greatest thermal gradients reach their maximum value in the porcelain at the respective ends of the metal sheath I9, whereas the mechanical stresses on the porcelain reach their maximum value in the bulge I6 where the gaskets 20 are urged thereagainst. By having the end portions of the sheath I9 extend beyond the region engaged by the gaskets 20, as shown, the maximum thermal gradients and maximum mechanical stresses are prevented from occurring at the same place.

As indicated in the next preceding paragraph when soft metal is used for the sheath, the metal will flow and equalize the stresses to which the porcelain is subjected due to the porcelain not being straight or true as a result of the method of forming and firing such porcelains. However, an important feature of the invention resides in the fact that the metal plating or sheath of the porcelain makes it possible to straighten or true the plated or sheathed pcrcelains by a common lathe operation. Ordinarily, of course, such turning or similar operation would be necessary only where a bent or untrue porcelain has been plated with a hard metal. However, it is to be understood that if round desirable the copper plating or sheathing referred to may be machined or turned down to true it to make accurate and intimate contact with the gaskets 2|] whereby localized pressures are avoided and the rate of heat transfer of the metal parts is substantially improved since the plated metal or girth sheath I9 contacts the porcelain over an extensive area.

The invention provides in a simple and inexpensive manner for improving the efticiency and lengthening the useful life oi spark plugs subject to extreme heat, and achieves the other advantages set out in the foregoing statement of objects.

Modification may be resorted to without departing from the spirit of the invention or the scope thereof as defined by the appended claims.

We claim:

1. A spark plug of the character described comprising a generally tubular metal shell formed interiorly with a circumferential recess, a porce lain insulator positioned axially within said shell and formed intermediate its ends with a circumferential bulge that extends into said recess, metal sheathing on the porcelain covering said bulge and extending above and below the same, and metal gaskets in said shell recess having wedging engagement with the metal sheathing in the region of said bulge.

2. A spark plug as defined in claim l in which the metal sheathing is composed of a soit metal that is readily deformable under stress.

3. A spark plug as dened in claim 1 in which the metal sheathing is composed of copper.

4. A spark plug as dened in claim 1 in which the metal sheathing is plated onto the porcelain.

5. A spark plug as defined in claim l in which the metal gaskets are of softer metal than the shell.

6. A spark plug as defined in claim l in which the metal of the gaskets comprises copper.

7. A spark plug comprising a porcelain insu lator for supporting a center electrode, a seamless girth sheath of soft metal on said porcelain, a metal shell formed interiorly with a circumferential recess in which said porcelain insulator is axially positioned, and metal gaskets occupying the remainder of said recess and engaging said sheath to constitute a, heat-conducting structure from the insulator to said shell.

8. A spark plug comprising a porcelain insulator for supporting a center electrode, a seamless girth sheath of copper plating on said porcelain,

. a metal shell formed interiorly with a circumferential recess in which said porcelain insulator is axially positioned, and metal gaskets occupying the remainder of said shell recess and having wedging engagement with the sheath on said insulator.

9. A spark plug as defined in claim 1 in which the metal sheathing is of a type having substantially the same low thermal expansion as Dorcelain.

10. A spark plug comprising a porcelain insulator for supporting a center electrode, and a seamless girth sheath of metal on said porcelain, said metal having substantially the same low thermal expansion as porcelain.

MELVILLE F. PETERS. JOHN ELLSWORTH D. MCCARTY. 

